The present invention is directed to means and methods for automatically adjusting currents in shim coils for improving homogeneity of the main magnet field for nuclear magnetic resonance (NMR) instruments. The term "shimming" in this art denotes the process of compensating inhomogeneities in the main magnet field. The name can be traced back to the time when large resistive electromagnets were used for NMR. Coarse adjustments were done by placing thin brass pieces between the magnet and the poles in order to align the pole faces. The metal pieces employed were shim stock, and thus the prooedure was called "shimming". Today a shim set is a combination of electromagnets within the magnet's bore, each of which is designed to produce a specific magnetic field profile. In principle, the maximum achievable field homogeneity is limited only by the number of these shims, whereas in practice the winding tolerances are the limiting factor.
The homogeneity requirements span a wide range. Several tens of parts per million (ppm) deviation are acceptable for clinical imaging if it can be maintained over the cross-section of a body, while a few parts per billion (pph) in some milliliters are required in high-resolution spectroscopy.
Coarse, permanent field errors due to shielding in the magnet's environment and due to manufacturing imperfections should be countered by superconducting shim coils. They are adjusted after the magnet has been brought up to field and do not consume power afterwards. Changes in the environment and susceptibility variations of the sample itself must be compensated by room-temperature shim coils. Adjusting the currents in these coils can be a time-consuming procedure when done by hand, since it traditionally involves repetitive observation of the free induction decay FID signal and/or its lineshape. The shim currents can be iteratively adjusted following a fixed protocol, until the apparently "best" line shape is found.
Ideally, automatic shimming by a computer should yield a specific homogeneity over a desired region in a fraction of the time required for a human operator to achieve the same homogeneity. The hardware requirements are computer-controllable shim power supplies. Conventional autoshim software emulates to some extent the way a human operator would proceed. A start-up sequence follows a fixed protocol to bring up the field such as from a totally unadjusted state. Then a search algorithm is invoked which changes the settings one at a time by different step values. A measure of field quality is computed from the incoming FID signal, from which the program determines whether or not to proceed in that direction or to try another change. It is hoped in the prior art that such a "blind search" will converge after a number of iterations to an acceptable field optimum. NMR studies benefit greatly from on-line field adjustment, for instance the "ppm vs. volume" figure could be optimized differently in the course of an experiment. The possibility of selecting a region of interest within the sample by "focussing" a homogeneous field onto this area would provide much better spectral resolution and signal-to-noise ratio.
A first problem with the prior art approach is that, since the FID signal is the volume integral over the entire sample, it is not known what parts of the field need to be adjusted. Secondly, it is difficult to define a "figure-of-merit" which describes the quality of the signal. A popular measure is the total area under the FID signal, but other optimization criteria may be preferable, depending on the application. Thirdly, all search algorithms are inherently slow since they must wait after every change for the coil currents to settle before the next scan can be started. Fourthly, if the set of applicable shim-sets is not orthogonal over the volume of the sample, then it is necessary to perform a large number of iterations. Finally, there is no guarantee that the search will find the globally best result. Experience shows that iterative searches fail on complex, second-order interactions between the shim components, as discussed further below.
This disclosure is with reference to the following publications as background within which the present invention arises.
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Romeo and Hoult (6) have discussed the general framework with which any field may be analyzed in terms of spherical harmonic components, and they describe the theoretical advantages to be gained by considering zonal and tesseral components separately. In practice it is desired to analyze as rapidly as possible a particular field in terms of a given set of shimming gradients, each of which may deviate from the single, respective component it was designed to ideally simulate.